Superplastic behaviour in nano ceramics A. Domínguez-Rodríguez University of Seville (Spain)

1. Superplastic behaviour in nano ceramicsA. Domínguez-RodríguezUniversity of Seville (Spain)

2. *Definition macro and micro of superplasticity*Equation of superplasticity *How to improve superplasticity*Superplasticity in nano-ceramics: nano-MgO nano-YTZP

3. 3YTZP deformed at 1450 ºC and 3x10-4 s-1 (Courtesy to Prof. F. Wakai)

4. A grain switching event observed during superplastic deformation of Y-TZP. A group of grains exchange their neighbors during deformation. (Courtesy to Prof. R. Duclos)

5. In a material superplastically deformed:*The deformation is due to grain boundary sliding*The strain rate is controlled by the accomodation process: -Diffusion of point defects -Activity of dislocations -Cavities

6. Equation of superplasticity is the strain rate σ is the applied stress σ0 is the threshold stress n and p the stress and grain size exponents Q is an activation energy

7. How to improve superplasticityThe strategy to enhance superplasticity is twofold:*Refinement of the microstructure*Improvement of the accommodation processAlthough both processes are independent each other, in many cases are connected.

8. Superplasticity in nano-MgO

9. Grain size distribution from the nc-MgO showing the log-normal distribution with mean grain diameter of 37 nm

10. Nano-MgO superplastically deformed

11. These nano-MgO could be deformed in compression, at temperatures between 700 and 800 ºC at strain rates around 10-5 s-1 and strains around 40 %. Values of the stress exponent, n = 2, and the activation energy of 200 kJ/mol were obtained for all test conditions.Very small grain sizes permit diffusional processes to vary from slow lattice diffusion to a much faster grain boundary one and to allow grains to reach a significant mobility.

12. Superplasticity in nano-YTZP

13. In the case of YTZP, it has been successively shown that Y3+segregates at grain boundaries, inducing a local electric field which is screened by the gradient of oxygen vacancies between the bulk and the boundaries.When the grain size of the polycrystal becomes close to the screening length (nanoscale length), this electric field can influence the diffusional processes and in consequence the creep behavior of the nano YTZP.

14. Position (nm) Yttrium segregation assessed -----

15. Constitutive equation for nano YTZP

16. Where V(R) is the electric potential at the grain boundaries, zD is the effective electric charge of the diffusing cations, and  is the screening lenght (Debye length) and εr is thedielectricconstant of the material.

17. Plot of  versus grain size for different  values

18. Final remarksIt is clear that the refinement of the microstructure can improve superplasticity in nano-MgO but not in nano-YTZP due to the nature of the grain boundary in this ceramics.In conclusion: to improve superplasticity it is more important to control the nature of the grain boundaries that the grain size itself.